Can you wait and buy it when you arrive? NO
If you plan on being independent, travelling a lot, or want to know where you are in relation to everything else, you will need maps. You can buy maps in Uganda, but only in very large shops, and most of those are in Kampala. It is much better to acquire maps before you leave home.
Getting your maps before you leave is essential, of course, if youwant to plan your trip in advance and plot places of interest, palces to stay etc.
In the UK we are used to the excellent quality OS map series, which outshines most other countries, including the USA. In Uganda maps of such detail if they exist, will be in military hands, and beyond your reach.
General route planning maps are available, but don't expect 1:50,000 or 1:25,000 sheets - they don't exist.